I think the higher you go, the more of an effect coaching has. I also would not be surprised (although I haven't looked) if the difference between the top 15 teams was closer in D1A than D1AA, which is closer than D2/D3. The difference being at D2/D3 the top teams have greater vision, allowing them to see better players. The rich get richer. In the higher divisions, where everyone has basically the same set of recruits to fight over, coaching (including both game planning and practice plans) has a correspondingly greater role.